1. Field of the Invention
The invention relates to an intake air amount estimation apparatus/method for an internal combustion engine.
2. Description of the Related Arts
An amount of intake air supplied into cylinders must be grasped in order to control an air-fuel ratio. It has been known that the amount of intake air changes in accordance with the pressure in an intake pipe disposed downstream of a throttle valve. A related art for approximating and calculating an intake air amount by means of a linear equation of an intake pipe pressure is disclosed. In related arts disclosed in Japanese Patent Application Laid-Open No. 2002-180877, the fact that an intake air amount decreases because of backflow of exhaust gas in cylinders into an intake pipe during a suction stroke if valve overlap is caused by a variable valve timing mechanism is taken into account in approximation based on a linear equation of an intake pipe pressure. In addition, the fact that a decrease in intake air amount is reduced due to the unlikelihood of backflow of exhaust gas if the intake pipe pressure is equal to or higher than a predetermined pressure is also taken into account.
That is, an intake air amount is calculated on the basis of an intake pipe pressure using a linear equation that differs depending on whether or not valve overlap has been caused. If valve overlap has been caused, an intake air amount is calculated using two continuous linear equations that are replaced with each other when the intake pipe pressure reaches a predetermined value.